This invention pertains generally to an improved, sero-controlled, hydraulic valve assembly and particularly to a valve assembly of a type comprising a directional valve, such as a spool valve, and a pilot valve controlling the directional valve. This invention pertains additionally to a valve structure for use in the pilot valve and to a flow restrictor for use in the directional valve.
An examwple of a valve assembly of the type noted above, wherein a directional, four-way, spool valve is controlled by a pilot valve, is disclosed in U.S. Pat. No. 3,943,957. The spool valve is a closed center valve in one embodiment and an open center valve in another embodiment. Another example is disclosed in U.S. Pat. No. 4,050,476, in which the spool valve is a closed center valve, and which also discloses an example of a known type of flow restrictor for use in such an assembly. The same type of slow restrictor is further disclosed in U.S. Pat. Re. 29,714, originally U.S. Pat. No. 3,688,800.
Prior valve assemblies in which such a directional valve is controlled by a pilot valve have a number of shortcomings, most of which are attributable to the pilot valve. Prior pilot valves employing feedback pressures (see, for examwple, U.S. Pat. No. 3,943,957) tend to react erroneously to feedback pressures which are due to leakage of hydraulic fluid in the directional valve or to hysteresis in the motor used to operate the pilot valve. Furthermore, such pilot valves tend to be highly sensitive to any temperature or viscosity changes in hydraulic fluid. Consequently, in a valve assembly employing such a pilot valve to control a directional valve, the directional valve tends undesirably to drift out of its center or null condition.